Deliberate mistake: Sydney's real harbour skyline has some neon signs on top of buildings. In the movie all them are omitted, presumably for advertising and aesthetic purposes.
Factual error: Crush the sea turtle says he's 150 years old and "still young." Actually, at 150, Crush would be almost twice the normal lifespan of a sea turtle, which is about 80 years. Contrary to popular belief, not all kinds of turtles have lifespans of over 100 years. It is tortoises, not turtles, that are known for their very long lifespans and, even then, the current record is 188 years old, not much older than Crush.
Continuity mistake: After Dory and Marlin passed the jellyfish and Marlin passes out, when the movie returns to the tank, the camera pans over the tank from right to left, and there's a reflection of the Sydney skyline. However, neither walls nor window frames show in this reflection. (00:45:55)
Factual error: When Bruce is chasing Marlin and Dory inside the submarine, the chase goes through a few 90 degree turns, but each turn that leads them to chase width-ways inside the submarine is far longer than the actual dimensions of the submarine would allow.
Character mistake: In the scene where the dentist is doing a root canal, Bloat says "rubber dam and clamp installed?" and another fish replies "yes", but when the picture goes back to the dentist and patient you can plainly see that there is not a rubber dam on the patient. A rubber dam is a large piece of latex attached to a metal frame, to keep water from the dental handpiece from going into the patient's mouth. Although all of the dental jargon in the film is precisely accurate, this device would have been very noticeable.
Audio problem: When Marlin and Dory swim into the deepest part of the ocean to reclaim their mask, they see a light (which we know later, belongs to an angler fish). They stop right underneath the light and says "What is it?" but his mouth doesn't match the movements. It looks as though the animators had Albert Brooks (Marlin's voice actor) say a different line than what was animated.
Continuity mistake: Jacques wakes up Nemo sleeping in his hut with a bunch of rocks. Nemo gets up and starts floating above the rocks. In the next shot, from the back of the hut, Nemo is still floating in the same spot, but the rocks have vanished.
Factual error: Clown fish are born as hermaphrodites and when a dominant female is found they become male. They only become female when their female mate is dead/gone. Then a hermaphrodite clown fish will become male and be the new female's mate and the process starts all over again. Coral should not have been the same size as Marlin; she should have been bigger, since clown fish that turn female will then grow bigger than their mate if they aren't already. (00:01:00)
Continuity mistake: When the tank becomes clean again, Peach reads the Aqua Scum 2003 manual that is on the counter. Later on, in the close-up of Nemo rolling in the plastic bag, the manual is not on the counter. A few shots later, it's back.
Continuity mistake: When Nigel flies into the dentist's office, he nudges one of the picture frames, and it moves a little. Two shots later, it's back to its original position.
Continuity mistake: When Nigel flies into the dentist's office, he knocks one of the picture frames falls on the floor. Later on, while the dentist is trying to catch Nigel, the picture frame disappears.
Continuity mistake: When the dentist grabs Nigel, the window blinds are slightly down. Then when he throws him out the window, the blinds are suddenly up all the way.
Continuity mistake: When Nemo is first seen in the fish tank, he gets scared and swims into the little diving helmet, then swims out of it. The other fish start talking to him, and in the next shot, he is suddenly much closer to the diving helmet than he was in the last shot.
Continuity mistake: When the dentist shoos Nigel away from the window, the picture frames on the counter fall over. In the next shot, two of them move closer to the fish tank, and one of them moves closer to the tray.
Continuity mistake: When Nigel flies into the window, there is a picture of a man on the counter. In the next shot of it, the picture changes position.
Continuity mistake: When Marlin and Dory get to the trench, there are some rocks around them. In the next shot, they disappear.
Continuity mistake: Decorations inside the fish tank often disappear, appear in random places, or change directions and spots throughout the film.
Continuity mistake: When Marlin and Dory are looking at the boats trying to find the one that took Nemo, they swim past a boat that says "Jerome's Raft." on it. When Nigel swoops down and gets them, the boat is completely missing, and the boat that was next to it is now the one closest to the dock.
